1. Field of the Invention
The present invention relates generally to the treatment of conditions involving undesired or pathological levels of inducible nitric oxide synthase (iNOS), e.g. septic shock or neuroinflammatory diseases. In one important aspect, the invention relates to methods of suppressing, inhibiting or preventing the accumulation of nitric-oxide induced cytotoxicity by using inhibitors that block or suppress the induction of cytokines and/or inducible nitric oxide synthase. Another aspect of the invention is the treatment of conditions involving undesired or pathological levels of proinflammatory cytokines (i.e. TNF-α, IL-1β, IL-2, IL-6, IL-8 and/or IFN-γ) and/or iNOS. One important aspect of the invention relates to methods of suppressing, inhibiting, or preventing proinflammatory cytokines and/or iNOS induced or aggravated disorders including conditions involving the detrimental effects of inflammation (e.g. disorders such as lupus, rheumatoid arthritis, osteoarthritis, amyotrophic lateral sclerosis, and autoimmune disorders; ischemia/reperfusion; neuroinflammatory conditions such as Alzheimer's, stroke, multiple sclerosis, X-linked adrenoleukodystrophy; and the effects of aging).
2. Description of Related Art
Nitric Oxide and Proinflammatory Cytokines
Nitric oxide (NO) is a potent pleiotropic mediator of physiological processes such as smooth muscle relaxation, neuronal signaling, inhibition of platelet aggregation and regulation of cell mediated toxicity. It is a diffusible free radical which plays many roles as an effector molecule in diverse biological systems including neuronal messenger, vasodilation and antimicrobial and antitumor activities (Nathan, 1992; Jaffrey et al., 1995). NO appears to have both neurotoxic and neuroprotective effects and may have a role in the pathogenesis of stroke and other neurodegenerative diseases and in demyelinating conditions (e.g. multiple sclerosis, experimental allergic encephalopathy, X-adrenoleukodystrophy) and in ischemia and traumatic injuries associated with infiltrating macrophages and the production of proinflamatory cytokines (Mitrovic et al., 1994; Bo et al., 1994; Merrill et al., 1993; Dawson et al., 1991, Kopranski et al., 1993; Bonfoco et al., 1995). A number of pro-inflammatory cytokines and endotoxin (bacterial lipopolysaccharide, LPS) also induce the expression of iNOS in a number of cells, including macrophages, vascular smooth muscle cells, epithelial cells, fibroblasts, glial cells, cardiac myocytes as well as vascular and non-vascular smooth muscle cells. Although monocytes/macrophages are the primary source of iNOS in inflammation, LPS and other cytokines induce a similar response in astrocytes and microglia (Hu et al., 1995; Galea et al., 1992).
During inflammation, reactive oxygen species (ROS) are generated by various cells including activated phagocytic leukocytes; for example, during the neutrophil “respiratory burst”, superoxide anion is generated by the membrane-bound NADPH oxidase. ROS are also believed to accumulate when tissues are subjected to inflammatory conditions including ischemia followed by reperfusion. Superoxide is also produced under physiological conditions and is kept in check by superoxide dismutates. Excessively produced superoxide overwhelms the antioxidant capacity of the cell and reacts with NO to form peroxynitrite, ONOO−, which may decay and give rise to hydroxyl radicals, OH (Marietta, M., 1989; Moncada et al., 1989; Saran et al., 1990; Beckman et al. 1990). NO, peroxynitrite and OH are potentially toxic molecules to cells including neurons and oligodendrocytes that may mediate toxicity through modification of biomolecules including the formation of iron-NO complexes of iron containing enzyme systems (Drapier et al., 1988), oxidation of protein sulfhydryl groups (Radi et al., 1991), nitration of proteins and nitrosylation of nucleic acids and DNA strand breaks (Wink et al., 1991).
There is now substantial evidence that iNOS plays an important role in the pathogenesis of a variety of diseases. In addition, it is now thought that excess NO production may be involved in a number of conditions, including conditions that involve systemic hypotension such as septic and toxic shock and therapy with certain cytokines. Circulatory shock of various etiologies is associated with profound changes in the body's NO homeostasis. In animal models of endotoxic shock, endotoxin produces an acute release of NO from the constitutive isoform of nitric oxide synthase in the early phase, which is followed by induction of iNOS. NO derived from macrophages, microglia and astrocytes has been implicated in the damage of myelin producing oligodendrocytes in demyelinating disorders like multiple sclerosis and neuronal death during neuronal degenerating conditions including brain trauma (Hu et al., 1995; Galea et al., 1992; Koprowski et al., 1993; Mitrovic et al., 1994; Bo et al., 1994; Merrill et al., 1993).
NO is synthesized from L-arginine by the enzyme nitric oxide synthase (NOS) (Nathan, 1992). Nitric oxide synthases are classified into two groups. One type, constitutively expressed (cNOS) in several cell types (e.g., neurons, endothelial cells), is regulated predominantly at the post-transcriptional level by calmodulin in a calcium dependent manner (Nathan, 1992; Jaffrey et al., 1995). In contrast, the inducible form (iNOS), synthesized de novo in response to different stimuli in various cell types including macrophages, hepatocytes, myocytes, neutrophils, endothelial and messangial cells, is independent of calcium. Astrocytes, the predominant glial component of brain have also been shown to induce iNOS in response to bacterial lipopolysaccharide (LPS) and a series of proinflammatory cytokines including interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ) (Hu et al., 1995; Galea et al., 1992).
Cytokines associated with extracellular signaling are involved in the normal process of host defense against infections and injury, in mechanisms of autoimmunity and in the pathogenesis of chronic inflammatory diseases. It is believed that nitric oxide (NO), synthesized by nitric oxide synthetase (NOS) mediates deleterious effects of the cytokines (Nathan, 1987; Zang et al., 1993; Kubes et al., 1991). For example, NO as a result of stimuli by cytokines (e.g., TNF-α, IL-1 and interleukin-6 (IL-6) is implicated in autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, osteoarthritis (Zang et al., 1993; McCartney-Francis et al., 1993). The NO produced by iNOS is associated with bactericidal properties of macrophages (Nathan, 1992; Stuehr et al., 1989). Recently, an increasing number of cells (including muscle cells, macrophages, keratinocytes, hepatocytes and brain cells) have been shown to induce iNOS in response to a series of proinflammatory cytokines including IL-1, TNF-α, interferon-γ (IFN-γ) and bacterial lipopolysaccharides (LPS) (Zang et al., 1993; Busse et al., 1990; Genge et al., 1995).
Signal Transduction Pathways
Mevalonate metabolites, particularly farnesyl pyrophosphate (FPP), are involved in post-translational modification of some G-proteins, including Ras (Goldstein et al., 1990; Casey et al., 1989). The inhibition of isoprenylation of Ras proteins by inhibitors of mevalonate pathway and their membrane association and transduction of signal from Ras to Raf/MAP kinase cascade (Kikuchi et al., 1994) indicates a role of mevalonate metabolites in the transduction of signal from receptor tyrosine kinases to Raf/MAP kinase cascade. Two enzymes that control the rate-limiting steps of the mevalonate pathway are 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, which catalyzes the formation of mevalonate from acetyl-CoA, and mevalonate pyrophosphate decarboxylase, which controls the use of mevalonate within the cell by converting 3-phospho-5-pyrophospho-mevalonate to isopentenyl pyrophosphate. Lovastatin, a potent inhibitor of HMG-CoA reductase, and sodium salt of phenylacetic acid (NaPA), an inhibitor of mevalonate pyrophosphate decarboxylase, are known to reduce the level of cellular isoprenoids (Castillo et al., 1991; Samid et al., 1994) and isoprenylated proteins (Repko and Maltese, 1989). No suppression of isoprenylated protein maturation in vitro by lovastatin treatment that produced 50% inhibition of sterol biosynthesis has been observed (Sinensky et al., 1991). The IC50 for inhibition of sterol synthesis is 10 nM, whereas the IC50 for inhibition of conversion of pro-p21ras to mature-p21ras is maximal at 2.6 μM (Sinensky et al., 1991). The pharmacologically attainable concentration for NaPA, however, is 1 to 5 mM (Thibault et al., 1995). HMG-CoA reductase can also be inhibited by 5-amino 4-imidazolecarboxamide ribotide (AICAR). AICAR stimulates AMP-activated protein kinase, an enzyme that inhibits acetyl-CoA carboxylase and HMG-CoA reductase (Henin et al., 1995)
LPS is shown to bind cell-surface receptor CD14 (Stefanova et al., 1993) and induce iNOS, probably via activation of NFkβ (Xie et al., 1994; Kwon et al., 1995). NFkβ is an ubiquitous multisubunit transcription factor that is activated in response to various stimuli including cytokines TNF-α, IL-1, IL-2, IL-6, viruses, LPS, DNA damaging agents and phorbol myristate acetate (PMA) (Schreck et al., 1992). Previous studies (Law et al., 1992) demonstrating the inhibition of NF-kβ activation by mevinolin and 5′-methylthioadenosine indicates a role of protein farnesylation and carboxyl methylation reactions in the activation of NF-kβ. Identification of the binding site of NF-kβ in the promoter region of the iNOS gene and that the activation of NFkβ in LPS-induced iNOS induction establishes a role of NFkβ activation in the induction of iNOS (Xie et al., 1994). Although the precise mechanism of NFkβ activation is not known at the present time, the inhibition of activation of NFkβ by inhibitors of tyrosine kinase and proteases indicates a role of phosphorylation and degradation of Ikβ in this process (Menon et al., 1993; Henkel et al., 1993).
Reactive oxygen (Schreck et al., 1992) and reactive nitrogen (Lander et al., 1993) species have been demonstrated to mediate the signal for NFkβ activation. The differential induction of NFkβ by protein phosphatase inhibitors in primary and transformed cell lines also indicates that induction of NFkβ is dependent on the dual processes of cellular redox and phosphorylation (Menon et al., 1993). The exact target of ROS that modulate cellular redox is unknown, and the lack of induction in cells in which activity of p21ras was blocked through expression of a dominant negative mutant or treatment with farnesyltransferase inhibitor indicate that direct activation of p21ras may be the central mechanism by which redox stress stimuli transmit its signal to the nucleus (Lander et al., 1995).
Ceramide Production and Apoptosis
Cytokine-mediated ceramide production is implicated in apoptosis of different cells including brain cells (Brugg et al., 1996; Wiesner and Dawson, 1996). Several studies support a role for hydrolysis of sphingomyelin as a stress-activated signaling mechanism in which ceramide plays a role in cell regulation, cell differentiation, growth suppression and apoptosis in various cell types including glial and neuronal cells (Hannun and Bell, 1989; Hannun, 1994; Kolesnick and Golde, 1994; Brugg et al., 1996; Wiesner and Dawson, 1996). Sphingomyelin is preferentially concentrated in the outer leaflet of the plasma membrane of most mammalian cells; it comprises sphingosine (a long chain sphingoid base backbone), a fatty acid, and a phosphocholine head group. Ceramide is composed of a sphingoid base with a fatty acid in amide linkage. Ceramide activates the proteases of the interleukin-converting enzyme (ICE) family (especially prICE/YAMA/CPP32), the protease responsible for cleavage of poly(A)DP-ribose polymerase (Martin et al., 1995), and that the activation of prICE by ceramide and induction of apoptosis are inhibited by overexpression of Bcl-2 (Zhang et al., 1996). Addition of exogenous ceramides or sphingomyelinase to cells induces stress-activated protein kinase-dependent transcriptional activity through the activation of c-jun (Latinis and Koretzky, 1996), and a dominant negative mutant of SEK1, the protein kinase responsible for phosphorylation and activation of stress-activated protein kinase, interferes with ceramide-induced apoptosis (Verheij et al., 1996). These observations also indicate that both Bcl-2 and stress-activated protein kinase function downstream of ceramide in the apoptotic pathway.
The signaling events in cytokine-mediated activation of sphingomyelin degradation to ceramide are poorly understood. Since the discovery of the sphingomyelin cycle, several inducers have been shown to be coupled to sphingomyelin-ceramide signaling events, including 1α,25-dihydroxyvitamin D3, radiation, antibody cross-linking, TNF-α, IFN-γ, IL-1β, nerve growth factor, and brefeldin A (Hannun and Bell, 1989; Hannun, 1994; Kolesnick and Golde, 1994; Zhang and Kolesnick, 1995; Kantey et al., 1995; Linardic et al., 1996).
The sphingomyelin pathway-associated signal transduction pathway mediates the action of several extracellular stimuli that lead to important biochemical and cellular effects (Zhang and Kolesnick, 1995; Kantey et al., 1995; Yao et al., 1995; Hannun, 1996; Lozano et al., 1994). In the case of TNF-α, the pathway is initiated by the action of TNF-α on its 55-kDa receptor, leading to phospholipase A2 activation, generation of arachidonic acid, and subsequent activation of sphingomyelinase (Jayadev et al., 1994). This pathway is initiated by the activation of two distinct forms of sphingomyelinase (SMase), a membrane-associated neutral sphingomyelinase (Chatterjee, 1993) and an acidic sphingomyelinase (Spence, 1993), which reside in the caveola and the endosomal-lysosomal compartment. Each type of SMase hydrolyzes the phosphodiester bond of sphingomyelin to yield ceramide and phosphocholine. Proinflammatory cytokines (tumor necrosis factor-α, TNF-α; interleukin-1β, IL-1β; interferon-γ, IFN-γ) and bacterial lipopolysaccharides have been shown as potent inducers of SMases. Ceramide has emerged as a second messenger molecule that is considered to mimic most of the cellular effects of cytokines and lipopolysaccharide in terminal differentiation, apoptosis, and cell cycle arrest (Zhang and Kolesnick, 1995; Kantey et al., 1995).
Sphingomyelin turnover and ceramide generation in response to TNF-α and IL-1β, occurs within minutes of stimulation; however, the sequence of events linking receptor stimulation and SMase activation remains largely unknown (Hannun, 1996; Lozano et al., 1994; Jayadev et al., 1994). In a number of cell systems, interaction of TNF-α with its membrane receptors (p75 and p55) has been found to activate phospholipase A2 and to induce release of arachidonic acid from phosphatidylcholine and phosphatidylethanolamine pools. This arachidonic acid has been shown as a mediator of sphingomyelin hydrolysis in response to TNF-α (Jayadev et al., 1994). In addition, proteases have also been implicated in the pathway leading from TNF-α to the activation of SMase (Hannun, 1996; Dbaio et al., 1997) recently. On the other hand, IL-1β and TNF-α are known to induce the production of reactive oxygen species, a class of highly diffusible and ubiquitous molecules, which have been suggested to act as second messengers (Tiku et al., 1990; Lo and Cruz, 1995; Devary et al., 1991). ROS encompassing species such as superoxide, hydrogen peroxide, and hydroxyl radicals are known to regulate critical steps in the signal transduction cascade and many important cellular events including protein phosphorylation, gene expression, transcription factor activation, DNA synthesis, and cellular proliferation (Schreck et al., 1991; Sen and Packer, 1996). A recent observation has shown that glutathione or similar molecules inhibit the activity of magnesium-dependent neutral SMase in vitro (Liu and Hannun, 1997). However, surprisingly, the SH group of GSH was not required as S-methyl GSH and GSSG inhibited neutral SMase at lower concentrations than GSH (Liu and Hannun, 1997). On the other hand, N-acetylcysteine has also been found to inhibit the synthesis of ceramide in cultured rat hepatocytes through the inhibition of dihydroceramide desaturase (Michel et al., 1997).
Inflammatory Diseases
NO generated by iNOS has been implicated in the pathogenesis of inflammatory diseases. In experimental animals hypotension induced by LPS or TNF-alpha can be reversed by NOS inhibitors and reinitiated by L-arginine (Kilbourn et al., 1990). Conditions which lead to cytokine-induced hypotension include septic shock, hemodialysis (Beasley and Brenner, 1992) and IL-2 therapy in cancer patients (Hibbs et al., 1992). Studies in animal models have suggested a role for NO in the pathogenesis of inflammation and pain and NOS inhibitors have been shown to have beneficial effects on some aspects of the inflammation and tissue changes seen in models of inflammatory bowel disease (Miller et al., 1990) and cerebral ischemia and arthritis (Ialenti et al., 1993; Stevanovic-Racic et al., 1994).
Inflammation, iNOS activity and/or cytokine production has been implicated in a variety of diseases and conditions, including psoriasis (Ruzicka et al., 1994; Kolb-Bachofen et al., 1994; Bull et al., 1994); uveitis (Mandia et al., 1994); type 1 diabetes (Eisieik & Leijersfam, 1994; Kroncke et al., 1991; Welsh el al., 1991); septic shock (Petros et al., 1991; Thiemermann & Vane, 1992; Evans et al., 1992; Schilling et al., 1993); pain (Moore et al., 1991; Moore et al, 1992; Meller et al., 1992; Lee et al., 1992); migraine (Olesen et al., 1994); rheumatoid arthritis (Kaurs & Halliwell, 1994); osteoarthritis (Stadler et al., 1991); inflammatory bowel disease (Miller et al., 1993; Miller et al., 1993); asthma (Hamid et al., 1993; Kharitonov et al., 1994); Koprowski et al., 1993); immune complex diseases (Mulligan et al., 1992); multiple sclerosis (Koprowski et al., 1993); ischemic brain edema (Nagafuji et al., 1992; Buisson et al., 1992; Trifiletti et al., 1992); toxic shock syndrome (Zembowicz & Vane, 1992); heart failure (Winlaw et al., 1994); ulcerative colitis (Boughton-Smith et al., 1993); atherosclerosis (White et al., 1994); glomerulonephritis (Muhl et al., 1994); Paget's disease and osteoporosis (Lowick et al., 1994); inflammatory sequelae of viral infections (Koprowski et al., 1993); retinitis, (Goureau et al., 1992); oxidant induced lung injury (Berisha et al., 1994); eczema (Ruzica et al., 1994); acute allograft rejection (Devlin, J. et al., 1994); and infection caused by invasive microorganisms which produce NO (Chen, Y and Rosazza, J. P. N., 1994).
In the central nervous system, apoptosis may play an important pathogenetic role in neurodegenerative diseases such as ischemic injury and white matter diseases (Thompson, 1995; Bredesen, 1995). Both X-linked adrenoleukodystrophy (X-ALD) and multiple sclerosis (MS) are demyelinating diseases with the involvement of proinflammatory cytokines in the manifestation of white matter inflammation. The presence of immunoreactive tumor necrosis factor a (TNF-α) and interleukin 1 (IL-1β) in astrocytes and microglia of X-ALD brain has indicated the involvement of these cytokines in immunopathology of X-ALD and aligned X-ALD with MS, the most common immune-mediated demyelinating disease of the CNS in man (Powers, 1995; Powers et al., 1992; McGuiness et al., 1995; McGuiness et al., 1997). Several studies demonstrating the induction of proinflammatory cytokines at the protein or mRNA level in cerebrospinal fluid and brain tissue of MS patients have established an association of proinflammatory cytokines (TNF-α, IL-1β, IL-2, IL-6, and IFN-γ) with the inflammatory loci in MS (Maimone et al., 1991; Tsukada et al., 1991; Rudick and Ransohoff, 1992).
X-linked adrenoleukodystrophy (X-ALD), an inherited, recessive peroxisomal disorder, is characterized by progressive demyelination and adrenal insufficiency (Singh, 1997; Moser et al., 1984). It is the most common peroxisomal disorder affecting between 1/15,000 to 1/20,000 boys and manifests with different degrees of neurological disability. The onset of childhood X-ALD, the major form of X-ALD, is between the age of 4 to 8 and then death within the next 2 to 3 years. Although X-ALD presents as various clinical phenotypes, including childhood X-ALD, adrenomyeloneuropathy (AMN), and Addison's disease, all forms of X-ALD are associated with the pathognomonic accumulation of saturated very long chain fatty acids (VLCFA) (those with more than 22 carbon atoms) as a constituent of cholesterol esters, phospholipids and gangliosides (Moser et al., 1984) and secondary neuroinflammatory damage (Moser et al., 1995). The necrologic damage in X-linked adrenoleukodystrophy may be mediated by the activation of astrocytes and the induction of proinflammatory cytokines. Due to the presence of similar concentration of VLCFA in plasma and as well as in fibroblasts of X-ALD, fibroblasts are generally used for both prenatal and postnatal diagnosis of the disease (Singh, 1997; Moser et al., 1984).
The deficient activity for oxidation of lignoceroyl-CoA ligase as compared to the normal oxidation of lignoceroyl-CoA in purified peroxisomes isolated from fibroblasts of X-ALD indicated that the abnormality in the oxidation of VLCFA may be due to deficient activity of lignoceroyl-CoA ligase required for the activation of lignoceric acid to lignoceroyl-CoA (Hashmi et al., 1986; Lazo et al., 1988). While these metabolic studies indicated lignoceroyl-CoA ligase gene as a X-ALD gene, positional cloning studies led to the identification of a gene that encodes a protein (ALDP), with significant homology with the ATP-binding cassette (ABC) of the super-family of transporters (Mosser et al., 1993). The normalization of fatty acids in X-ALD cells following transfection of the X-ALD gene (Cartier et al., 1995) supports a role for ALDP in fatty acid metabolism; however, the precise function of ALDP in the metabolism of VLCFA is not known at present.
Similar to other genetic diseases affecting the central nervous system, the gene therapy in X-ALD does not seem to be a real option in the near future and in the absence of such a treatment a number of therapeutic applications have been investigated (Singh, 1997; Moser, 1995). Adrenal insufficiency associated with X-ALD responds readily with steroid replacement therapy, however, there is as yet no proven therapy for neurological disability (Moser, 1995). Addition of monoenoic fatty acid (e.g., oleic acid) to cultured skin fibroblasts of X-ALD patients causes a reduction of saturated VLCFA presumably by competition for the same chain elongation enzyme (Moser, 1995). Treatment of X-ALD patients with trioleate resulted in 50% reduction of VLCFA. Subsequent treatment of X-ALD patients with a mixture of trioleate and trieruciate (popularly known as Lorenzo's oil) also led to a decrease in plasma levels of VLCFA (Moser, 1995; Rizzo et al., 1986; Rizzo et al., 1989). Unfortunately, the clinical efficacy has been unsatisfactory since no proof of favorable effects has been observed by attenuation of the myelinolytic inflammation in X-ALD patients (Moser, 1995). Moreover, the exogenous addition of unsaturated VLCFA induces the production of superoxide, a highly reactive oxygen radical, by human neutrophils (Hardy et al., 1994). Since cerebral demyelination of X-ALD is associated with a large infiltration of phagocytic cells to the site of the lesion (Powers et al., 1992), treatment with unsaturated fatty acids may even be toxic to X-ALD patients. Bone marrow therapy also appears to be of only limited value because of the complexity of the protocol and of insignificant efficacy in improving the clinical status of the patient (Moser, 1995).
Experimental allergic encephalomyelitis (EAE) is an inflammatory demyelinating disease of the central nervous system (CNS) that serves as a model for the human demyelinating disease, multiple sclerosis (MS). Studies have shown that the majority of the inflammatory cells constitute of T-lymphocytes and macrophages (Merrill and Benveniste, 1996). These effector cells and astrocytes have been implicated in the disease pathogenesis by secreting number of molecules that act as inflammatory mediators and/or tissue damaging agents such as nitric oxide (NO). NO is a molecule with beneficial as well as detrimental effects. In neuroinflammatory diseases like EAE, high amounts of NO produced for longer durations by inducible nitric oxide synthase (iNOS) acts as a cytotoxic agent towards neuronal cells. Previous studies have shown NO by itself or it's reactive product (ONOO−) may be responsible for death of oligodendrocytes, the myelin producing cells of the CNS, and resulting in demyelination in the neuroinflammatory disease processes (Merrill et al., 1993; Mitrovic et al., 1994).
Infiltrating T-lymphocytes in EAE produce pro-inflammatory cytokines such as IL-12, TNF-α and IFN-γ (Merrill and Benveniste, 1996). In addition to T-cells and macrophages, astrocytes have also been shown to produce TNF-α (Shafer and Murphy, 1997). Convincing evidence exists to support a role for both TNF-α and IFN-γ in the pathogenesis of EAE (Taupin et al., 1997; Villarroya et al., 1996; Issazadeh et al., 1995). Investigations with antibodies against TNF-α have shown that in mice these antibodies protect against active and adaptively transferred EAE disease (Klinkert et al., 1997). The expression of TNF-α and IFN-γ during EAE disease could result in the upregulation of iNOS in macrophage and astrocytes because TNF-α and IFN-γ have been shown to be potent inducers of iNOS in macrophages and astrocytes in culture (Xie et al., 1994). This induction of iNOS could result in the production of NO, which if produced in large amounts may lead to cytotoxic effects. Peroxynitrite (ONOO−) has been identified in both MS and EAE CNS (Hooper et al., 1997; van der Veen et al., 1997). The role of peroxynitrite in the pathogenesis of EAE is supported by the beneficial effects of uric acid, a peroxynitrite scavenger, against EAE and by a subsequent survey documenting that MS patients had significantly lower serum uric acid levels than those of controls (Hooper et al., 1998). However, aggravation of EAE by inhibitors of NOS activity (Ruuls et al., 1996) and in an animal model of iNOS gene knockout (Fenyk-Melody et al., 1998) indicate that NO may not be the only pathological mediator in EAE disease process. In addition to NO other free radicals such as reactive oxygen intermediates (O2−, H2O2, and OH−) can also be stimulated by cytokines (Merrill and Benveniste, 1996). Reactive oxygen intermediates (ROI) and NO are believed to be key mediators of pathophysiological changes that take place during inflammatory disease process. ROI's such as superoxide anion, hydroxy radicals and hydrogen peroxide can also be stimulated by TNF-α (Merrill and Benveniste, 1996). Therefore, it is likely that both the direct modulation of cellular functions by proinflammatory cytokines and toxicity of the ROI and reactive nitrogen species may play a role in the pathogenesis of EAE disease.
Several studies on protein and/or mRNA levels in plasma, cerebrospinal fluid (CSF), brain tissue, and cultured blood leukocytes from MS patients have established an association of proinflammatory cytokines (TNF-α, IL-1 and IFN-γ) with MS (Taupin et al., 1997; Villarroya et al., 1996; Issazadeh et al., 1995). The mRNA for iNOS has also been detectable in both MS as well as EAE brains (Bagasra et al., 1995; Koprowski et al., 1993). Semiquantitative RT-PCR™ for iNOS mRNA in MS brains shows markedly higher expression of iNOS mRNA in MS brains than control brains (Bagasra et al., 1995). Analysis of CSF from MS patients has also shown increased levels of nitrite and nitrate compared with normal control (Merrill and Benveniste, 1996). Peroxynitrite, ONOO— is a strong nitrosating agent capable of nitrosating tyrosine residues of proteins to nitrotyrosine. Increased levels of nitrotyrosine have been found in demyelinating lesions of MS brains as well as spinal cords of mice with EAE (Hooper et al., 1998; Hooper et al., 1997). A strong correlation exists between CSF levels of cytokines, disruption of blood-brain barrier, and high levels of circulating cytokines in MS patients (Villarroya et al., 1996; Issazadeh et al., 1995). Increase in TNF-α and IFN-γ levels seems to predict relapse in MS and the number of circulating IFN-γ positive blood cells correlates with severity of disability. These observations support the view that in both MS and EAE, induction of proinflammatory cytokines and production of NO through iNOS play roles in the pathogenesis of these diseases.
Alzheimer's disease (AD) is the most common degenerative dementia affecting primarily the elderly population. The disease is characterized by the decline of multiple cognitive functions and a progressive loss of neurons in the central nervous system. Deposition of beta-amyloid peptide has also been associated with AD. Over the last decade, a number of investigators have noted that AD brains contain many of the classical markers of immune mediated damage. These include elevated numbers of microglia cells, which are believed to be an endogenous CNS form of the peripheral macrophage, and astrocytes. Of particular importance, complement proteins have been immunohistochemically detected in the AD brain and they most often appear associated with beta-amyloid containing pathological structures known as senile plaques (Rogers et al., 1992; Haga et al., 1993).
These initial observations which suggest the existence of an inflammatory component in the neurodegeneration observed in AD has been extended to the clinic. A small clinical study using the nonsteroidal anti-inflammatory drug, indomethacin, indicated that indomethacin significantly slowed the progression of the disease (Neurology, 43(8):1609 (1993)). In addition, a study examining age of onset among 50 elderly twin pairs with onsets of AD separated by three or more years, suggested that anti-inflammatory drugs may prevent or delay the initial onset of AD symptoms (Neurology, 44:227 (1994)).
Over the years numerous therapies have been tested for the possible beneficial effects against EAE or MS disease but with mixed results (Cross et al., 1994; Ruuls et al., 1996). Though aminoguandine (AG) has been described as a competitive inhibitor of iNOS and a suppressor of its expression (Corbett and McDaniel, 1996; Joshi et al., 1996), to date few compounds which inhibit iNOS are of potential therapeutic value have been identified. This deficiency is particularly troubling given the significant cellular damage which can arise as a result of iNOS-mediated nitric oxide toxicity, especially in chronic inflammatory disease states. There is a present need for therapeutic agents which will inhibit or even prevent cytotoxic concentrations of NO from occurring in individuals suffering from diseases and conditions to which NO toxicity or an undesired production of proinflammatory cytokines is linked.